Hi,
Try at first to fix the applet settings.
Right Mouse Click -> Edit Connections -> Wireless
Delete the connections except for yours.

Try to connect (restart the modem, or replug the wireless tub, or restart the computer).
If the error persists try the following:
Left Mouse Click -> Connect to Hidden Wireless -> Connection
and select from there your connection.

If the keyring appears on your connection clear and reset the keyrings:
$ rm ~/.gnome2/keyrings/default.keyring

Hi,
Connect the LAN & ping to wireless device. It may happen that someone has
change the passphrase of wifi network key after putting details on Ubuntu
wifi. If not then checkout following commands: Note before using read the
man pages:

iwlist,
iwevent,
iwconfig,
iwgetid,
iwspy,
iwpriv.

still having the problem then delete the wifi entry from
network-manager-applet & reenter correctly.
Good Luck.